1. Field of the Invention
The present invention relates to stereo sound image enhancement, and more particularly concerns methods and apparatus for enhancing directivity of left and right channel sounds produced by a stereo speaker system.
2. Description of Related Art
In my prior U.S. Pat. No. 4,748,669 for Stereo Enhancement System, there is described a stereo sound image enhancement system in which sum and difference signals are processed so as to boost certain frequency components of the difference signal and to relatively attenuate certain frequency components of the sum signal. In addition, amplitude of the processed difference signal is servo controlled so as to maintain a relatively constant amount of stereo sound from one record to another or from one time to another within a given record.
Certain applications of the enhancement system and method of my prior patent, and many other stereo sound systems also, can benefit considerably from increased directivity of the stereo sound image. By increased directivity is meant such selective enhancement of sound from one side or the other of the apparent stereo sound image that exaggerates or amplifies sound that appears to emanate from one side or the other of the stereo sound image or from an area displaced from the center of the image. For example, where different sound elements of a source, such as particular instruments, are positioned at fixed locations to one side or the other of the center stage, it is desirable, in reproduction of such sound source, to emphasize or enhance the fact that such individual instrument is at its particular location, a location that is displaced from the center of the audio image. Moreover, it is desired to expand the apparent width of the entire sound image to provide an enhanced stereo sound. With such an arrangement of enhanced directivity, not only is apparent lateral displacement of individual instruments and other fixed sound sources enhanced, but the subjective effect of motion created by sound of an object moving from right to left or left to right across the sound field of the listener is also greatly increased. For example, when watching a highspeed automobile or airplane moving across a television or movie screen from right to left, the viewer not only sees the vehicle crossing the screen, but also hears the sound of the vehicle approaching the right side of the screen before the visual image appears on the screen. Sound from the left is of lower intensity at this time. So too, as the vehicle moves to the left edge of the screen and beyond view, sound from the left side of the stereo sound image increases, and sound from the right side decreases. Initially, with a vehicle moving from right to left, the vehicle sound appears to come solely from the right. As the vehicle exits at the left side of the screen, vehicle sound appears to come only from the left. By suitably and controllably magnifying the sound primarily appearing to come from the right side and sound primarily appearing to come from left side of the stereo sound image, the total subjective effect of the combined visual and audible motion from right to left is greatly enhanced. In other words, directivity of the sound image is increased. Even in the absence of any visual image, apparent motion of the audio image is more realistic with enhancement of directivity. No such directivity enhancement, whether for moving sound images or for laterally displaced fixed sound source components, is available in any prior art insofar as applicant is aware.
Accordingly, it is an object of the present invention to provide directivity enhancement for a stereo sound image.